1. Field of the Invention
This invention generally relates to nondestructive evaluation of a material, and more specifically to surface photo acoustic wave measurement to determine thickness or other properties of a material used in semiconductor device fabrication.
2. Related Art
During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects, titanium and tantalum as adhesion-promoting layers, and titanium nitride and tantalum nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of metal films vary depending on their function: Conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick.
Metal films are typically deposited and patterned in complex geometries in the microprocessor. A geometry currently used in microelectronics fabrication is a xe2x80x9cdamascenexe2x80x9d or xe2x80x9cdual damascenexe2x80x9d structure. Damascene-type structures, used primarily to form copper conductors and interconnects, are typically formed by a multi-step process: i) an oxide layer on a wafer is first etched to have a series of trenches and then coated with a diffusion-barrier layer of tantalum or tantalum nitride; ii) copper is electrolytically plated onto the wafer to fill the coated trenches; iii) the structure is then mechanically polished to remove excess copper, leaving only trenches filled with the diffusion-barrier layer and copper. The resulting structure is a series of separated copper lines having a thickness of a few thousand angstroms, a width and periodicity of about 0.5 microns, and a length of several millimeters.
Non-metal thin films also have considerable application in low dielectric constant (k) applications such as optical coatings, sensors, and insulating films for use in ULSI circuit devices. Silica aerogel films are of particular interest. The porosity and density of the insulating film are difficult to measure but are directly related to the dielectric constant (k). Young""s modulus is another important property to be measured that is also correlated with k.
During typical fabrication processes, films are deposited to have a thickness of within a few percent (e.g., 5-100 angstroms) of their target value. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor""s fabrication. Noncontact, nondestructive measurement techniques (e.g., optical techniques) are preferred because they can measure patterned xe2x80x9cproductxe2x80x9d samples, (e.g., damascene samples) rather than xe2x80x9cmonitorxe2x80x9d samples. Measurement of product samples accurately indicates errors in fabrication processes and additionally reduces costs associated with monitor samples.
One optical technique for film-thickness measurements uses a single, short (100 fs) optical pulse to generate an acoustic pulse that propagates into a multilayer structure. The acoustic pulse reflects off various interfaces (i.e., layer/layer and substrate/layer interfaces) in the structure, thus causing its echo to return to the structure""s surface. The returning echo pulse modulates the surface reflectivity and is measured with a variably delayed optical probe pulse. The thickness of the layers in the structure is determined by analyzing the time dependence of the reflected probe beam and the sound velocities of the film and substrate materials.
In another prior technique, a measurement system launches a surface acoustic wave with known wavelength xcex. A fixed wavelength is imprinted on the wave by illuminating the copper surface with a powerful pulsed laser. The laser beam is divided into multiple beams so that an array of alternating light regions (constructive interference) and dark regions (destructive interference) as above. The period of the array is the imprinted wavelength. The copper is heated and expands in the region of the light stripes, and surface acoustic waves are launched in the two opposite directions perpendicular to the stripes. The surface acoustic wave is a series of ripples on the surface and effectively forms a diffraction grating. A second laser beam is diffracted off the grating and the surface acoustic wave frequency is imprinted on this second beam. The wave frequency is measured by the time dependence of the diffracted beam intensity. If the frequency is measured by the system, then the wave speed c can be calculated. Once c is known, the film thickness can be determined if the material acoustic constants are known.
A first aspect of the invention is a method for determining the thickness, density or other properties of a material that involves producing an acoustic wave at a first frequency in a material layer with a first laser beam. After the wave with a known frequency is created within the material layer, the angle of diffraction of a second laser beam from the acoustic wave is measured. With the measured angle of diffraction and the known frequency of the wave, the wavelength of the acoustic wave and thickness of the material layer or layers are then determined.
Another aspect of the invention is a system for measuring the thickness, density or other properties of a material. The system comprises a first laser, the first laser creating a first beam, the first beam creating an acoustic wave at a first frequency in the material, a second laser, the second laser creating a second beam, a portion of which is reflected and a portion of which is diffracted by the material, and a position sensing detector that measures the angle of diffraction of the diffracted portion of the second beam.